High-efficiency solar cell structures and methods of manufacture

ABSTRACT

Solar cells of varying composition are disclosed, generally including a central substrate, conductive layer(s), antireflection layers(s), passivation layer(s) and/or electrode(s). Multifunctional layers provide combined functions of passivation, transparency, sufficient conductivity for vertical carrier flow, the junction, and/or varying degrees of anti-reflectivity. Improved manufacturing methods including single-side CVD deposition processes and thermal treatment for layer formation and/or conversion are also disclosed.

RELATED APPLICATION INFORMATION

This Application claims the benefit of previously filed U.S. ProvisionalApplication entitled “High-Efficiency Solar Cell Structures and Methodsof Manufacture,” filed 21 Apr. 2009 and assigned application No.61/171,194, which Application is hereby incorporated by reference hereinin its entirety. All aspects of the present invention may be used incombination with any of the disclosures of the above-noted Application.

TECHNICAL FIELD

The present invention relates to solar cells. More particularly, thepresent invention relates to improved solar cell structures and methodsof their manufacture for increased cell efficiency.

BACKGROUND OF THE INVENTION

Solar cells are providing widespread benefits to society by convertingessentially unlimited amounts of solar energy into useable electricalpower. As their use increases, certain economic factors becomeimportant, such as high-volume manufacturing and efficiency.

High volume manufacturing is generally considered to attain a highdegree of cost-effectiveness and efficiency if the number ofmanufacturing steps, and the complexity of each step, can be minimized.

Finished solar cell efficiencies of 20% or more are highly desired inthe industry, however, known embodiments of such efficient cells oftensuffer from cell structure complexity and/or manufacturing complexity.

What is required therefore, are solar cells which attain highoperational efficiency, and which can be manufactured in a costeffective manner.

SUMMARY OF THE INVENTION

The shortcomings of the prior art are overcome and additional advantagesare provided by the present invention which in one aspect extends to anyone or a combination of the solar cell structures disclosed below,including generally a central substrate, conductive layer(s),antireflection layers(s), passivation layer(s) and/or electrode(s).Multifunctional layers provide combined functions of passivation,transparency, sufficient conductivity for vertical carrier flow, thejunction, and/or varying degrees of anti-reflectivity. Improvedmanufacturing methods including single-side CVD deposition processes andthermal treatment for layer formation and/or conversion are alsodisclosed.

In one aspect the present invention includes methods of fabricating anyof these structures, including: providing a wafer as a centralsubstrate; deposition or growth of interface passivation layers over thesubstrate; deposition of conductive layers over the passivation layers;providing thermal treatment; optional deposition of antireflectivelayers (possibly including back side mirrors); and providingmetallization as electrodes.

In one embodiment the present invention comprises applying a heattreatment to produce a multifunctional film which separates into asurface passivating interface layer and a highly doped polycrystallinepassivation layer with high transparency.

In one embodiment the present invention comprises depositing anamorphous, silicon containing compound and using a heat treatment toinitiate crystallization into a polycrystalline film.

In one embodiment the present invention comprises depositing anamorphous, silicon containing compound and using a heat treatment whichleads to a crystallization of the film and increases the opticaltransmissivity.

In one embodiment the present invention comprises depositing anamorphous, silicon containing compound and using a heat treatment inorder to activate doping atoms in the compound.

In one embodiment, the present invention comprises depositing anamorphous, silicon containing compound and using a thermal treatmentgreater than 500° C. in order to activate doping atoms in the compoundand result in diffusion of dopant atoms into a substrate wafer toprovide a high-low junction or a p-n junction.

Systems and computer program products corresponding to theabove-summarized methods are also described and claimed herein.

Further, additional features and advantages are realized through thetechniques of the present invention. Other embodiments and aspects ofthe invention are described in detail herein and are considered a partof the claimed invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other objects, features, andadvantages of the invention are apparent from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is an energy band diagram for an n-type crystalline silicon solarcell with doped polysilicon layers and a passivated interface;

FIG. 2 is a partial cross-sectional view of a solar cell depicting onetype of minority and majority carrier flow for a front junction, p-typewafer;

FIG. 3 is a partial cross-sectional view of a solar cell depicting onetype of minority and majority carrier flow for a back junction, p-typewafer;

FIG. 4 is a partial cross-sectional view of a solar cell depicting onetype of minority and majority carrier flow for a front junction, n-typewafer;

FIG. 5 is a partial cross-sectional view of a solar cell depicting onetype of minority and majority carrier flow for a back junction, n-typewafer;

FIG. 6 is a partial cross-sectional view of a solar cell having n-typefront, n- or p-type wafer, and p-type back;

FIG. 7 is a partial cross-sectional view of a solar cell having n-typefront, n- or p-type wafer, p-type back, in a bifacial configuration;

FIG. 8 is a partial cross-sectional view of a solar cell having n-typefront, n-type wafer, p-type back, and includes isolating theantireflection coating;

FIG. 9 is a partial cross-sectional view of a solar cell having n-typefront, n-type wafer, p-type back, and includes a multifunctionaltransparent, conductive, highly doped silicon compound layer;

FIG. 10 is a partial cross-sectional view of a solar cell having n- orp-type wafer, n-type front, including certain front layer improvements,and p-type back;

FIG. 11 is a partial cross-sectional view of a solar cell having ap-type front, n- or p-type wafer, and n-type back;

FIG. 12 is a partial cross-sectional view of a solar cell having p-typefront, n- or p-type wafer, n-type back, in a bifacial configuration;

FIG. 13 is a partial cross-sectional view of a solar cell having p-typefront, p-type wafer, n-type back, and includes isolating theantireflection coating;

FIG. 14 is a partial cross-sectional view of a solar cell having p-typefront, p-type wafer, n-type back, and including a multifunctionaltransparent, conductive, highly doped silicon compound layer;

FIG. 15 is a partial cross-sectional view of a solar cell having n- orp-type wafer, p-type front, including certain front layer improvements,and n-type back;

FIG. 16 is a partial cross-sectional view of a solar cell having glassor other transparent film with embedded electrodes, compressed or bondedto the cell;

FIG. 17 is a partial cross-sectional view of a solar cell having glassor other transparent film with embedded electrodes, compressed or bondedto the cell, including a localized electrode on the back; and

FIG. 18 is a partial cross-sectional view of a solar cell havingadditional silicon buffer layers formed therein;

all in accordance with the present invention.

DESCRIPTION OF THE INVENTION

With reference to the energy band diagram and partial cross-sectionalviews of exemplary solar cells of FIGS. 1-5, solar radiation is assumedto preferentially illuminate one surface of a solar cell, usuallyreferred to as the front side. To achieve high energy conversionefficiency of incident photons into electric energy, an efficientabsorption of photons within the silicon substrate material forming thecell is important. This can be achieved by a low parasitic opticalabsorption of photons within all layers except the substrate itself.

For the sake of simplicity, the geometrical surface shape of layersurfaces (e.g., surface texture such as pyramids, or other surfacetexture, can be formed on layer surfaces) is not portrayed in thesedrawings, however, it is understood that the geometrical shape and/orsurfaces may be textured in any shape beneficial for improved solar cellefficiency, and falls within the scope of the invention.

One important parameter for high solar cell efficiency is surfacepassivation. Surface passivation provides suppression of recombinationof electrons and holes at or in the vicinity of certain physicalsurfaces within the solar cell. Surface recombination can be reduced bythe application of dielectric layers. These layers reduce the interfacedensity of states and therefore reduce the number of recombinationcenters. Two examples are thermally grown silicon oxide and PECVDdeposited silicon nitride. Another example of a surface passivatinglayer is intrinsic amorphous silicon. These layers can also provide anelectrical charge which reduces the number of carriers of the oppositepolarity and reduces recombination rates via this mechanism. Twoexamples are silicon nitride and aluminum oxide.

Another method of reducing the amount of carriers of one type close to asurface is the diffusion of doping atoms either of the same or theopposite doping of the layer doping type. In this case doping levels inexcess of the layer doping are necessary to obtain a high-low junction(also commonly called back surface field or front surface field) or ap-n junction. This can be combined with other methods of surfacepassivation mentioned above.

Surface passivation can play an important role in achieving highlyefficient solar cells. In most of the solar cell structures describedbelow in accordance with the present invention, multiple layers ormultifunctional layers can provide excellent surface passivation. Thiscan be achieved via a very steep doping profile and an additionalpassivation of the interface using a layer with low interface statedensity and a high band gap resulting in a tunneling barrier forsubstrate minority carriers to pass. A corresponding energy band diagramis shown in FIG. 1. The solid lines depict the case of an n-typecrystalline silicon wafer with a passivated interface and dopedpolycrystalline silicon passivation layers. The dotted lines representthe case of an n-type crystalline silicon wafer and a double layerstructure of intrinsic amorphous silicon followed by doped amorphoussilicon layer, sometimes referred to as a heterojunction cell.

These structures provide another benefit for a highly efficient solarcell: the recombination in areas underneath the contacts can be as lowas in areas without contacts. The contacts can be shielded by thepassivation. As a result, the contact area can be optimized for opticalproperties, thus minimizing resistive losses, but carrier recombinationis decoupled.

Depending on the choice of materials, and doping types andconcentrations, the disclosed cell structures may be categorized asfront junction or back junctioncells. In a front junction cell theminority carriers (in the case of a p-type wafer these are electrons)are collected on the side of illumination. In a back junction cell theminority carriers are collected at the side opposed to the illumination.Current flow patterns are shown generally in the partial cross-sectionalsolar cell views of FIGS. 2-5 for p-type and n-type wafers.

FIG. 2 shows carrier flow for a solar cell 20, in which minoritycarriers (solid lines) flow to front electrodes 21 from a p-type wafer25 having a front junction. The electrons need to use lateral flowwithin the thin n-type emitter 22 to reach the electrodes 21, and thelateral sheet resistance of the emitter 22 increases resistive losses.The majority carriers (dashed lines) can use the shortest geometricalpath to the full area back electrode 29.

FIG. 3 shows carrier flow for a solar cell 30 from p-type wafer 35having a back junction. The majority carriers (dashed lines) can use theentire wafer conductivity to reach the front electrodes 31. The minoritycarriers (solid lines) can use the shortest geometrical path to reachthe rear n-type emitter 38, and their transport within the emitter isvertical instead of mainly lateral. This back junction structure reducesthe requirements for lateral conductivity of the emitter layer.

FIG. 4 shows carrier flow for a solar cell 40, in which minoritycarriers (solid lines) flow to front electrodes 41 from an n-type wafer45 having a front junction. The holes need to use lateral flow withinthe thin p-type emitter 42 to reach the electrodes 41, and the lateralconductivity of the emitter determines the resistive losses. Themajority carriers (dashed lines) can use the shortest geometrical pathto the full area back electrode 49.

FIG. 5 shows carrier flow for a solar cell 50 from an n-type wafer 55having a back junction. The majority carriers (dashed lines) can use theentire wafer conductivity to reach the front electrodes 51. The minoritycarriers (solid lines) can use the shortest geometrical path to reachthe rear p-type emitter 58, and their transport within the emitter isvertical instead of mainly lateral. This back junction structure reducesthe requirements for lateral conductivity of the emitter layer.

A back junction cell with a full area back contact has the advantagethat the minority carriers do not have to flow laterally through theemitter to reach the contacts, their transport within the emitter ismainly vertical. This reduces the losses associated with the lateraltransport of the minority carriers within the emitter. Since the fullcontact area coverage is a requirement to benefit from this property ofthe structure, a shielded contact is important, e.g., since the metalcontacts the layer everywhere (“full area contact coverage”), there isno need for the minority carriers to flow laterally toward the nearestcontact, like they do within the emitter in, e.g., FIG. 4.

Exemplary Cell Structures: n-type front, n- or p-type wafer, p-typeback:

FIG. 6 is a partial cross-sectional view of a solar cell 60 having ann-type front, n- or p-type wafer, and p-type back.

The metal electrodes 61 and 69 are positioned on the outer layers 62 and68, respectively. This has the benefit that the metal does not need topenetrate underlying layers before it contacts the wafer. Furthermorethe silicon bulk wafer 65 is shielded from the contact interfaces andhence the contact interface carrier recombination is minimized. Thisstructure has an n-type front surface, which for a p-type wafer 65collects the minority carriers (electrons) on the front. Therefore amaximum lateral sheet resistance of, for example, 500 Ohm/sq of thecombined layers 62, 63 and 64 is required. For an n-type wafer thisstructure collects the minority carriers (holes) on the back. Thereforethe current flow pattern in the solar cell is different and therequirements of lateral conductivity of layer 62 are less critical.Exemplary layers of cell 60 include the following:

61: Front metal electrode.

62: Transparent and conductive film, refractive index in the range of1.4<n<3; thickness in the range of 20 nm <thickness<110 nm; sheetresistance of less than 500 Ohm/sq for a p-type wafer (front junctionsolar cell), specific resistivity in the range of rho<1000 Ohm cm for ann-type wafer (back junction solar cell). Examples include transparentconductive oxides like indium tin oxide, aluminum doped zinc oxide,fluorine doped tin oxide, tantalum oxide, antimony tin oxide, germaniumoxide, zirconium oxide, titanium oxide, gallium oxide, cadmium antimonyoxide.

63: Electrically passivating and conductive film, highly n-doped 1e18cm⁻³<N_(D)<5e21 cm⁻³; thickness in the range of 2 nm <thickness<50 nm;specific resistivity in the range of rho<1000 Ohm cm. Examples include:

-   -   n-type amorphous or polycrystalline silicon carbides: phosphorus        doped silicon carbide, nitrogen doped silicon carbide;    -   n-type amorphous or polycrystalline silicon: phosphorus doped        amorphous silicon, nitrogen doped amorphous silicon;    -   n-type amorphous or poly-crystalline diamond-like-carbon:        nitrogen doped diamond-like carbon.

Any of the above listed examples may include oxygen and hydrogen(n-doped SiC_(x)O_(y)H_(z); n-doped SiN_(x)O_(y)H_(z)).

64: Electrically passivating interface layer; thickness <10 nm; noconductivity requirements because of small thickness; no absorptionrestrictions due to small thickness. Examples include silicon oxide,silicon nitride, intrinsic amorphous silicon, intrinsic polycrystallinesilicon, aluminum oxide, aluminum nitride, phosphorus nitride, titaniumnitride.

65: n-type or p-type crystalline silicon wafer; thickness is the rangeof w<300 um, base resistivity for n-type wafers 0.5 Ohm cm<rho<20 Ohmcm, for p-type wafers 0.1 Ohm cm<rho<100 Ohm cm.

66: Electrically passivating interface layer; thickness <10 nm; noconductivity requirements because of small thickness; no absorptionrestrictions due to small thickness. Examples include silicon oxide,silicon nitride, intrinsic amorphous silicon, intrinsic polycrystallinesilicon, aluminum oxide, aluminum nitride, phosphorus nitride, titaniumnitride.

67: Electrically passivating and conductive film, highly p-doped1E18-5E21/cm³; specific resistivity in the range of rho<1000 Ohm cm.Examples include:

-   -   p-type amorphous or polycrystalline silicon carbides: boron        doped silicon carbide, aluminum doped silicon carbide, gallium        doped silicon carbide;    -   p-type amorphous or polycrystalline silicon: boron doped        silicon, aluminum doped silicon, gallium doped silicon;    -   p-type amorphous or poly-cystalline diamond-like-carbon: boron        doped diamond-like carbon, aluminum doped diamond-like carbon.

Any of the above examples may include oxygen and hydrogen (p-dopedSiC_(x)O_(y)H_(z); p-doped SiN_(x)O_(y)H_(z)).

68: transparent and conductive film, refractive index in the range of1.4<n<3; specific resistivity in the range of rho<1000 Ohm cm. Examplesinclude conductive oxides like indium tin oxide, aluminum doped zincoxide, fluorine doped tin oxide, tantalum oxide, antimony tin oxide,germanium oxide, zirconium oxide, titanium oxide, gallium oxide, cadmiumantimony oxide.

69: back metal electrode.

FIG. 7 is a partial cross-sectional view of a solar cell 70 havingn-type front, n- or p-type wafer, p-type back, in a bifacialconfiguration. Cell 70 is similar to cell 60 but includes localizedelectrodes 79 on the back. Because of the localized structure on theback, photons impinging from the rear of the solar cell can be absorbedwithin the wafer 75 and produce electron-hole pairs. This can increasethe power output generated by the solar cell under outdoor operatingconditions where albedo can be used at low additional modulemanufacturing and installation cost.

FIG. 8 is a partial cross-sectional view of a solar cell 80 havingn-type front, n-type wafer, p-type back, and includes isolating theantireflection coating. This structure is especially beneficial formaterial combinations where the conductive layers on the front surfaceof cell structures 60 and 70 have a high absorption. By placing theelectrode 81 directly on the contacting layer 83 the conductivityrequirements of layer 82 are waived and traditional antireflectioncoating films (which are insulators) can be used. Exemplary layers ofcell 80 include the following:

81: Front metal electrode.

82: Antireflection film, refractive index in the range of 1.4<n<3;thickness in the range of 20 nm<thickness<110 nm. Examples includesilicon nitride, silicon carbide, silicon oxide, transparent conductiveoxides.

83: Electrically passivating and conductive film; thickness<110 nm;highly n-doped 1e18 cm⁻³<N_(D)<5e21 cm⁻³, specific resistivity in therange of rho<1000 Ohm cm. Examples include:

-   -   n-type amorphous or polycrystalline silicon carbides: phosphorus        doped silicon carbide, nitrogen doped silicon carbide;    -   n-type amorphous or polycrystalline silicon: phosphorus doped        amorphous silicon, nitrogen doped amorphous silicon;    -   n-type amorphous or poly-crystalline diamond-like-carbon:        nitrogen doped diamond-like carbon.

Any of the above examples may include oxygen and hydrogen (n-dopedSiC_(x)O_(y)H_(z); n-doped SiN_(x)O_(y)H_(z)).

84: Electrically passivating interface layer; thickness thickness<10 nm;no conductivity requirements because of small thickness; no absorptionrestrictions due to small thickness. Examples include silicon oxide,silicon nitride, intrinsic amorphous silicon, intrinsic polycrystallinesilicon, aluminum oxide, aluminum nitride, phosphorus nitride, titaniumnitride.

85: n-type crystalline silicon wafer; thickness is in the range of w<300um, base resistivity for n-type wafers 0.5 Ohm cm<rho<20 Ohm cm.

86: Electrically passivating interface layer; thickness<10 nm; noconductivity requirements because of small thickness; no absorptionrestrictions due to small thickness. Examples include silicon oxide,silicon nitride, intrinsic amorphous silicon, intrinsic polycrystallinesilicon, aluminum oxide, aluminum nitride, phosphorus nitride, titaniumnitride.

87: Electrically passivating and conductive film, highly doped p-doped1e18 cm⁻¹<N_(A)<5e21 cm⁻; specific resistivity in the range of rho<1000Ohm cm. Examples include:

-   -   p-type amorphous or polycrystalline silicon carbides: boron        doped silicon carbide, aluminum doped silicon carbide, gallium        doped silicon carbide;    -   p-type amorphous or polycrystalline silicon: boron doped        silicon, aluminum doped silicon, gallium doped silicon;    -   p-type amorphous or poly-cystalline diamond-like-carbon: boron        doped diamond-like carbon, aluminum doped diamond-like carbon.

Any of the above examples may contain oxygen and hydrogen (p-dopedSiC_(x)O_(y)H_(z); p-doped SiN_(x)O_(y)H_(z)).

88: Transparent and conductive film, refractive index in the range of1.4<n<3; specific resistivity in the range of rho<1000 Ohm cm. Examplesinclude transparent conductive oxides like indium tin oxide, aluminumdoped zinc oxide, fluorine doped tin oxide, tantalum oxide, antimony tinoxide, germanium oxide, zirconium oxide, titanium oxide, gallium oxide,cadmium antimony oxide.

89: back metal electrode.

FIG. 9 is a partial cross-sectional view of a solar cell 90 havingn-type front, n-type wafer, p-type back, and including a multifunctionaltransparent, conductive, highly doped silicon compound layer. Thisaspect of the invention is an improvement to the other disclosuresabove, because the functions of layers 62 and 63 of, e.g., solar cell 60of FIG. 6 (and any other similar layers in any other embodimentsdisclosed herein) are combined into a multifunctional layer 93 adepicted in FIG. 9. This layer can be electrically passivating,transparent, and sufficiently conductive for a vertical carrier flow tothe electrodes (back junction solar cell), provides the junction withthe wafer 95 and/or reduces the reflectance of the incoming light (e.g.,antireflection coating). On the rear, layer 97 a can combine thefunctions of layers 67 and 68 of, e.g., solar cell 60 of FIG. 6 (and anyother similar layers in any other embodiments disclosed herein). Layer97 a provides the junction with the wafer 95, has a refractive indexwhich results in a high reflectivity for photons of more than 900 nmwavelength and is sufficiently conductive for vertical carrier flow fromthe wafer 95 to the metal electrode 99. Exemplary layers of cell 90include the following:

91: Front metal electrode.

93 a: Electrically passivating, transparent and conductive film,refractive index in the range of 1.4<n<3; thickness in the range of 20nm <thickness<110 nm; specific resistivity in the range of rho<1000 Ohmcm for an n-type wafer; highly doped n-doped 1e18 cm⁻³<N_(D)<5e21cm^(−3.). Examples include:

-   -   n-type amorphous or polycrystalline silicon carbides: phosphorus        doped silicon carbide, nitrogen doped silicon carbide;    -   n-type amorphous or polycrystalline silicon: phosphorus doped        amorphous silicon, nitrogen doped amorphous silicon;    -   n-type amorphous or poly-crystalline diamond-like-carbon:        nitrogen doped diamond-like carbon.

Any of the above examples may include oxygen and hydrogen (n-dopedSiC_(x)O_(y)H_(z); n-doped SiN_(x)O_(y)H_(z)).

94: Electrically passivating interface layer; thickness<10 nm; noconductivity requirements because of small thickness; no absorptionrestrictions due to small thickness. Examples include silicon oxide,silicon nitride, intrinsic amorphous silicon, intrinsic polycrystallinesilicon, aluminum oxide, aluminum nitride, phosphorus nitride, titaniumnitride.

95: n-type or p-type crystalline silicon wafer; thickness is in therange of w<300 um, base resistivity for n-type wafers 0.5 Ohm cm<rho<20Ohm cm, for p-type wafers 0.1 Ohm cm<rho<100 Ohm cm.

96: Electrically passivating interface layer; thickness<10 nm; noconductivity requirements because of small thickness; no absorptionrestrictions due to small thickness. Examples include silicon oxide,silicon nitride, intrinsic amorphous silicon, intrinsic polycrystallinesilicon, aluminum oxide, aluminum nitride, phosphorus nitride, titaniumnitride.

97 a: Electrically passivating and transparent and conductive film;specific resistivity in the range of rho<1000 Ohm cm. Examples include:

-   -   p-type amorphous or polycrystalline silicon carbides: boron        doped silicon carbide, aluminum doped silicon carbide, gallium        doped silicon carbide;    -   p-type amorphous or polycrystalline silicon: boron doped        silicon, aluminum doped silicon, gallium doped silicon;    -   p-type amorphous or poly-crystalline diamond-like-carbon: boron        doped diamond-like carbon, aluminum doped diamond-like carbon.

Any of the above examples may include oxygen and hydrogen (p-dopedSiC_(x)O_(y)H_(z); p-doped SiN_(x)O_(y)H_(z)).

99: back metal electrode.

FIG. 10 is a partial cross-sectional view of a solar cell 100 having n-or p-type wafer, n-type front, including certain front layerimprovements, and p-type back. The rear surface structures (omitted forconvenience) can be implemented according to any of the other structuresdescribed herein.

This structure is especially beneficial for material combinations wherethe layers x3 and x4 on, e.g., the front surface of structures disclosedabove, have unacceptably high absorption. (The x3 and x4 notation isfurther explained below and represents any of the above layers withreference numerals ending in 3,3 a, 4, 4 a, respectively). In cell 100,by placing layers 103 and 104 under the contacts only, their opticalproperties (refractive index, absorption) are not important for cellefficiency. Resistance losses only occur through vertical carrier flowto the contacts 101. Layers 102, 104 b and 105 b also do not have toshield the contact, so they can be optimized for transmissivity andsurface passivation. If they do provide lateral conductivity, this willfacilitate the current flow towards the contacts and the contactstructures can be placed further apart from each other. This reducesoptical shading losses. This structure works best with a back junctionsince the lateral conductivity requirements of layer 102, 104 b and 105b are waived. Exemplary layers of cell 100 include the following:

101: Front metal electrode.

102: Antireflection film, refractive index in the range of 1.4<n<3;thickness<150 nm. Examples include silicon nitride, silicon carbide,silicon oxide, titanium oxide, transparent conductive oxides.

103: Electrically passivating conductive film, e.g., thickness<50 nm;e.g., specific resistivity in the range of rho<1000 Ohm cm. Examplesinclude:

-   -   n-type amorphous or polycrystalline silicon carbides: phosphorus        doped silicon carbide, nitrogen doped silicon carbide;    -   n-type amorphous or polycrystalline silicon: phosphorus doped        amorphous silicon, nitrogen doped amorphous silicon;    -   n-type amorphous or poly-crystalline diamond-like-carbon:        nitrogen doped diamond-like carbon.

Any of the above examples may include oxygen and hydrogen (n-dopedSiC_(x)O_(y)H_(z); n-doped SiN_(x)O_(y)H_(z)).

104: Electrically passivating interface layer; thickness<10 nm; noconductivity requirements because of small thickness; no absorptionrestrictions due to small thickness. Examples include silicon oxide,silicon nitride, intrinsic amorphous silicon, intrinsic polycrystallinesilicon, aluminum oxide, aluminum nitride, phosphorus nitride, titaniumnitride.

104 b: Electrically passivating interface layer; thickness<110 nm.Examples include silicon oxide, silicon nitride, intrinsic amorphoussilicon, intrinsic polycrystalline silicon, aluminum oxide, aluminumnitride, phosphorus nitride, titanium nitride, silicon carbide or stacksof two or more of these materials.

105: n-type or p-type crystalline silicon wafer; thickness is in therange of w<300 um, base resistivity for n-type wafers 0.5 Ohm cm<rho<20Ohm cm, for p-type wafers 0.1 Ohm cm<rho<100 Ohm cm.

105 b: phosphorus diffused silicon layer (optional), sheet resistance>70Ohm/sq.

The structures above are not mutually exclusive, and any feature of onestructure, can apply to any other structure herein, in accordance withthe present invention.

Exemplary Cell Structures: p-type front, n- or p-type wafer, n-typeback:

FIG. 11 is a partial cross-sectional view of a solar cell 110 having ap-type front, n- or p-type wafer, and n-type back.

In this cell, the metal electrodes 111 and 119 are placed on the outerlayers 112 and 118, respectively. This provides the benefit that themetal does not need to penetrate underlying layers before it contactsthe wafer. Furthermore the silicon bulk wafer 115 is shielded from thecontact interfaces and hence the contact interface carrier recombinationis minimized. This structure has a p-type front surface, for an n-typewafer this structure collects the minority carriers (holes) on thefront. Therefore a maximum lateral sheet resistance of 500 Ohm/sq of thecombined layers 112, 113 and 114 is allowed. For a p-type wafer thisstructure collects the minority carriers (electrons) on the back.Therefore the current flow pattern in the solar cell is different andthe requirements on the lateral conductivity of layer 112 are lesscritical. Exemplary layers of cell 110 include the following:

111: Front metal electrodes.

112: Transparent and conductive film, refractive index in the range of1.4<n<3; thickness<110 nm; sheet resistance of less than 500 Ohm/sq foran n-type wafer, specific resistivity in the range of rho<1000 Ohm cmfor a p-type wafer. Examples include transparent conductive oxides likeindium tin oxide, aluminum doped zinc oxide, fluorine doped tin oxide,tantalum oxide, antimony tin oxide, germanium oxide, zirconium oxide,titanium oxide, gallium oxide, cadmium antimony oxide.

113: Electrically passivating and conductive film, highly doped p-doped1e18 cm⁻¹<N_(A)<5e21 cm⁻³; specific resistivity in the range of rho<1000Ohm cm. Examples include:

-   -   p-type amorphous or polycrystalline silicon carbides: boron        doped silicon carbide, aluminum doped silicon carbide, gallium        doped silicon carbide;    -   p-type amorphous or polycrystalline silicon: boron doped        silicon, aluminum doped silicon, gallium doped silicon;    -   p-type amorphous or poly-crystalline diamond-like-carbon: boron        doped diamond-like carbon, aluminum doped diamond-like carbon.

Any of the above examples may include oxygen and hydrogen (p-dopedSiC_(x)O_(y)H_(z); p-doped SiN_(x)O_(y)H_(z)).

114: Electrically passivating interface layer; <10 nm; no conductivityrequirements because of small thickness; no absorption restrictions dueto small thickness. Examples include silicon oxide, silicon nitride,intrinsic amorphous silicon, intrinsic polycrystalline silicon, aluminumoxide, aluminum nitride, phosphorus nitride, titanium nitride.

115: n-type or p-type crystalline silicon wafer; thickness is in therange of w<300 um, base resistivity for n-type wafers 0.5 Ohm cm<rho<20Ohm cm, for p-type wafers 0.1 Ohm cm<rho<100 Ohm cm.

116: Electrically passivating interface layer; thickness<10 nm; noconductivity requirements because of small thickness; no absorptionrestrictions due to small thickness. Examples include silicon oxide,silicon nitride, intrinsic amorphous silicon, intrinsic polycrystallinesilicon, aluminum oxide, aluminum nitride, phosphorus nitride, titaniumnitride.

117: Electrically passivating, transparent and conductive film, highlydoped n-doped 1e18 cm⁻³<N_(D)<5e21 cm⁻³; e.g., thickness in the range of2 nm<thickness<50 nm or more; specific resistivity in the range ofrho<1000 Ohm cm. Examples include:

-   -   n-type amorphous or polycrystalline silicon carbides: phosphorus        doped silicon carbide, nitrogen doped silicon carbide;    -   n-type amorphous or polycrystalline silicon: phosphorus doped        amorphous silicon, nitrogen doped amorphous silicon;    -   n-type amorphous or poly-crystalline diamond-like-carbon:        nitrogen doped diamond-like carbon.

Any of the above examples may contain oxygen and hydrogen (n-dopedSiC_(x)O_(y)H_(z); n-doped SiN_(x)O_(y)H_(z)).

118: Transparent and conductive film, refractive index in the range of1.4<n<3; specific resistivity in the range of rho<1000 Ohm cm. Examplesinclude transparent conductive oxides like indium tin oxide, aluminumdoped zinc oxide, fluorine doped tin oxide, tantalum oxide, antimony tinoxide, germanium oxide, zirconium oxide, titanium oxide, gallium oxide,cadmium antimony oxide.

119: back metal electrode.

FIG. 12 is a partial cross-sectional view of a solar cell 120 havingp-type front, n- or p-type wafer, n-type back, in a bifacialconfiguration. Cell 120 is similar to cell 110 but includes localizedelectrodes 129 on the back. Because of the localized structure on theback, photons impinging from the rear of the solar cell can be absorbedwithin the wafer 125 and produce electron-hole pairs. This can improvethe efficiency of the solar cell under outdoor operating conditionswhere albedo can be used at low additional module manufacturing andinstallation cost.

FIG. 13 is a partial cross-sectional view of a solar cell 130 havingp-type front, p-type wafer, n-type back, and includes isolating theantireflection coating. This structure is especially beneficial formaterial combinations where the conductive layers on the front surfaceof cell structures 110 and 120 have a high absorption. By placing theelectrodes 131 directly on the contacting layer 133 the conductivityrequirements of layer 132 are waived and traditional antireflectioncoating films (which are insulators) can be used. This structure worksbest with a back junction since the lateral conductivity requirements oflayer 133 and 134 are not critical. Exemplary layers of cell 130 includethe following:

131: Front metal electrodes.

132: Antireflection film, refractive index in the range of 1.4<n<3; <150nm. Examples include silicon nitride, silicon carbide, silicon oxide,aluminum oxide, titanium oxide, transparent conductive oxides.

133: Electrically passivating, transparent and conductive film;thickness<110 nm; specific resistivity in the range of rho<1000 Ohm cm.Examples include:

-   -   p-type amorphous or polycrystalline silicon carbides: boron        doped silicon carbide, aluminum doped silicon carbide, gallium        doped silicon carbide;    -   p-type amorphous or polycrystalline silicon: boron doped        silicon, aluminum doped silicon, gallium doped silicon;    -   p-type amorphous or poly-cystalline diamond-like-carbon: boron        doped diamond-like carbon, aluminum doped diamond-like carbon.

Any of the above examples may include oxygen and hydrogen (p-dopedSiC_(x)O_(y)H_(z); p-doped SiN_(x)O_(y)H_(z)).

134: Electrically passivating interface layer; thickness<10 nm; noconductivity requirements because of small thickness; no absorptionrestrictions due to small thickness. Examples include silicon oxide,silicon nitride, intrinsic amorphous silicon, intrinsic polycrystallinesilicon, aluminum oxide, aluminum nitride, phosphorus nitride, titaniumnitride.

135: p-type crystalline silicon wafer; thickness is the range of w<300um, base resistivity for p-type wafers 0.1 Ohm cm<rho<100 Ohm cm.

136: Electrically passivating interface layer; thickness<10 nm; noconductivity requirements because of small thickness; no absorptionrestrictions due to small thickness. Examples include silicon oxide,silicon nitride, intrinsic amorphous silicon, intrinsic polycrystallinesilicon, aluminum oxide, aluminum nitride, phosphorus nitride, titaniumnitride.

137: Electrically passivating, transparent and conductive film, highlydoped n-doped 1e18 cm⁻³<N_(D)<5e21 cm⁻³; specific resistivity in therange of rho<1000 Ohm cm. Examples include:

-   -   n-type amorphous or polycrystalline silicon carbides: phosphorus        doped silicon carbide, nitrogen doped silicon carbide;    -   n-type amorphous or polycrystalline silicon: phosphorus doped        amorphous silicon, nitrogen doped amorphous silicon;    -   n-type amorphous or poly-crystalline diamond-like-carbon:        nitrogen doped diamond-like carbon.

Any of the above examples may include oxygen and hydrogen (n-dopedSiC_(x)O_(y)H_(z); n-doped SiN_(x)O_(y)H_(z)).

138: transparent and conductive film, refractive index in the range of1.4<n<3; specific resistivity in the range of rho<1000 Ohm cm. Examplesinclude transparent conductive oxides like indium tin oxide, aluminumdoped zinc oxide, fluorine doped tin oxide, tantalum oxide, antimony tinoxide, germanium oxide, zirconium oxide, titanium oxide, gallium oxide,cadmium antimony oxide.

139: back metal electrode.

FIG. 14 is a partial cross-sectional view of a solar cell 140 havingp-type front, p-type wafer, n-type back, and including a multifunctionaltransparent, conductive, highly doped silicon compound layer. Thisaspect of the invention is an improvement to the other disclosuresabove, because the functions of layers 112 and 113 of, e.g., solar cell110 of FIG. 11 (and any other similar layers in any other embodimentsdisclosed herein) are combined into a multifunctional layer 143 adepicted in FIG. 14. This layer can be electrically passivating,transparent, and sufficiently conductive for a vertical carrier flow tothe electrodes (back junction solar cell), provides the junction withthe wafer 145 and/or reduces the reflectance of the incoming light(e.g., antireflection coating). On the rear, layer 147 a can combine thefunctions of layers 117 and 118 of, e.g., solar cell 110 of FIG. 11 (andany other similar layers in any other embodiments disclosed herein).Layer 147 a provides the junction with the wafer 145, has a refractiveindex which results in a high reflectivity for photons of more than 900nm wavelength and is sufficiently conductive for vertical carrier flowfrom the wafer 145 to the metal electrode 149. Exemplary layers of cell140 include the following:

141: Front metal electrode.

143 a: Electrically passivating and transparent and conductive film,refractive index in the range of 1.4<n<3; thickness<150 nm; specificresistivity in the range of rho<1000 Ohm cm. Examples include:

-   -   p-type amorphous or polycrystalline silicon carbides: boron        doped silicon carbide, aluminum doped silicon carbide, gallium        doped silicon carbide;    -   p-type amorphous or polycrystalline silicon: boron doped        silicon, aluminum doped silicon, gallium doped silicon;    -   p-type amorphous or poly-cystalline diamond-like-carbon: boron        doped diamond-like carbon, aluminum doped diamond-like carbon.

Any of the above examples may include oxygen and hydrogen (p-dopedSiC_(x)O_(y)H_(z); p-doped SiN_(x)O_(y)H_(z)).

144: Electrically passivating interface layer; thickness<10 nm; noconductivity requirements because of small thickness; no absorptionrestrictions due to small thickness. Examples include silicon oxide,silicon nitride, intrinsic amorphous silicon, intrinsic polycrystallinesilicon, aluminum oxide, aluminum nitride, phosphorus nitride, titaniumnitride.

145: n-type or p-type crystalline silicon wafer; thickness is in therange of w<300 um, base resistivity for n-type wafers 0.5 Ohm cm<rho<20Ohm cm, for p-type wafers 0.1 Ohm cm<rho<100 Ohm cm.

146: Electrically passivating interface layer; thickness<10 nm; noconductivity requirements because of small thickness; no absorptionrestrictions due to small thickness. Examples include silicon oxide,silicon nitride, intrinsic amorphous silicon, intrinsic polycrystallinesilicon, aluminum oxide, aluminum nitride, phosphorus nitride, titaniumnitride.

147 a: Electrically passivating and transparent and conductive film;specific resistivity in the range of rho<1000 Ohm cm for a highly dopedn-doped 1E18 cm⁻³<N_(D)<5E21 cm⁻³. Examples include:

-   -   n-type amorphous or polycrystalline silicon carbides: phosphorus        doped silicon carbide, nitrogen doped silicon carbide;    -   n-type amorphous or polycrystalline silicon: phosphorus doped        amorphous silicon, nitrogen doped amorphous silicon;    -   n-type amorphous or poly-crystalline diamond-like-carbon:        nitrogen doped diamond-like carbon.

Any of the above examples may include oxygen and hydrogen (n-dopedSiC_(x)O_(y)H_(z); n-doped SiN_(x)O_(y)H_(z)).

149: back metal.

FIG. 15 is a partial cross-sectional view of a solar cell 150 having n-or p-type wafer, p-type front, including certain front layerimprovements, and n-type back. The rear surface structures (omitted forconvenience) can be implemented according to any of the other structuresdescribed herein.

This structure is especially beneficial for material combinations wherethe layers xx3 and xx4 on, e.g., the front surface of structuresdisclosed above, have unacceptably high absorption. In cell 150, byplacing layers 153 and 154 under the contacts only, their opticalproperties (refractive index, absorption) are not important for cellefficiency. Resistance losses only occur through vertical carrier flowto the contacts 151. Layers 152, 154 b and 155 b also do not have toshield the contact, so they can be optimized for transmissivity andsurface passivation. If they do provide lateral conductivity, this willfacilitate the current flow towards the contacts and the contactstructures can be placed further apart from each other. This reducesoptical shading losses. This structure works best with a back junctionsince the lateral conductivity requirements of layer 152, 154 b and 155b are waived. Exemplary layers of cell 150 include the following:

151: Front metal electrode.

152: Antireflection film, refractive index in the range of 1.4<n<3;thickness thickness<110 nm. Examples include silicon nitride, siliconcarbide, silicon oxide, titanium oxide.

153: Electrically passivating conductive film, thickness<110 nm;specific resistivity in the range of rho<1000 Ohm cm. Examples include:

-   -   p-type amorphous or polycrystalline silicon carbides: boron        doped silicon carbide, aluminum doped silicon carbide, gallium        doped silicon carbide;    -   p-type amorphous or polycrystalline silicon: boron doped        silicon, aluminum doped silicon, gallium doped silicon;    -   p-type amorphous or poly-cystalline diamond-like-carbon: boron        doped diamond-like carbon, aluminum doped diamond-like carbon.

Any of the above examples may include oxygen and hydrogen (p-dopedSiC_(x)O_(y)H_(z); p-doped SiN_(x)O_(y)H_(z)).

154: Electrically passivating interface layer; thickness<10 nm; noconductivity requirements because of small thickness; no absorptionrestrictions due to small thickness. Examples include silicon oxide,silicon nitride, intrinsic amorphous silicon, intrinsic polycrystallinesilicon, aluminum oxide, aluminum nitride, phosphorus nitride, titaniumnitride.

154 b: Electrically passivating interface layer; thickness<10 nm.Examples include silicon oxide, silicon nitride, intrinsic amorphoussilicon, intrinsic polycrystalline silicon, aluminum oxide, aluminumnitride, phosphorus nitride, titanium nitride, silicon carbide.

155: n-type or p-type crystalline silicon wafer; thickness is in therange of w<300 um, base resistivity for n-type wafers 0.5 Ohm cm<rho<20Ohm cm, for p-type wafers 0.1 Ohm cm<rho<100 Ohm cm.

155 b: phosphorus diffused silicon layer (optional), sheet resistance>70Ohm/sq.

The structures above are not mutually exclusive, and any feature of onestructure, can apply to any other structure herein, in accordance withthe present invention.

Exemplary Cell Structures—Alternative Electrode Configurations:

FIG. 16 is a partial cross-sectional view of a solar cell 160 havingglass or other transparent film with embedded electrodes, compressed orbonded to the cell. This alternative applies to any of the structuresabove, and can include an n- or p-type front, n- or p-type wafer, and p-or n-type back. As an alternative to the metal electrodes beingdeposited directly on the cell, the metal electrodes 161 and 169 areembedded in a glass or other laminating films 161 a and 169 a. When theglass or laminating film is compressed or laminated under pressure, theembedded electrodes make contact on top of the outer layers 162 and 168,respectively. This has the benefit that the metal does not need to bedeposited directly onto the cell itself, thereby eliminating a typicalsource of film stress which can cause cell bowing. This is particularlyuseful when dealing with very large area wafers, such as thin-filmsilicon sheets and/or very thin wafers. In many of the embodimentsabove, the metal electrodes do no need to penetrate underlying layersbefore they contact the cell. Moreover, various conducing materials canbe used to enhance the electrical conductivity between the metalelectrodes 161 and 169 and the surface of the outer layers 162 and 168.These conducting materials could include but are not limited toanisotropic conducting films (ACF), conductive epoxies, or spring-likecontact probes. Exemplary layers of cell 160 include the following(which can be formed of any of the materials above, omitted here forsimplicity):

-   -   161 a: Glass plate or transparent film carrying embedded metal        electrodes.    -   161: Front metal electrode.    -   162: Transparent and conductive film.    -   163: Electrically passivating and conductive film.    -   164: Electrically passivating interface layer.    -   165: n-type or p-type crystalline silicon wafer; thickness is        the range of w<300 um.    -   166: Electrically passivating interface layer.    -   167: Electrically passivating and conductive film.    -   168: Transparent and conductive film.    -   169: Back metal electrode.    -   169 a: Glass plate or transparent film carrying embedded metal        electrode.

FIG. 17 is a partial cross-sectional view of a solar cell 170 havingglass or other transparent film with embedded electrodes, compressed orbonded to the cell, having localized electrodes 179 on the back. Becauseof the localized electrode structure on the back, photons impinging therear of the solar cell can be absorbed within the wafer 175 and produceelectron-hole pairs in this bifacial configuration. This can improve theefficiency of the solar cell under outdoor operating conditions wherealbedo can be used at low additional module manufacturing andinstallation cost.

This alternative applies to any of the structures above, and can includean n- or p-type front, n- or p-type wafer, and p- or n-type back. As analternative to the metal electrodes being deposited directly on thecell, the metal electrodes 171 and 179 are embedded in a glass or otherlaminating films 171 a and 179 a. When the glass or laminating film iscompressed or laminated under pressure, the embedded electrodes makecontact on top of the outer layers 172 and 178, respectively. This hasthe benefit that the metal does not need to be deposited directly ontothe cell itself, thereby eliminating a typical source of film stresswhich can cause cell bowing. This is particularly useful when dealingwith very large area wafers, such as thin-film silicon sheets and/orvery thin wafers. In many of the embodiments above, the metal electrodesdo no need to penetrate underlying layers before they contact the cell.Moreover, various conducing materials can be used to enhance theelectrical conductivity between the metal electrodes 171 and 179 and thesurface of the outer layers 172 and 178. These conducting materialscould include but are not limited to anisotropic conducting films (ACF),conductive epoxies, or spring-like contact probes. Exemplary layers ofcell 170 include the following (which can be formed of any of thematerials above, omitted here for simplicity):

-   -   171 a: Glass plate or transparent film carrying embedded metal        electrodes.    -   171: Front metal electrode.    -   172: Transparent and conductive film.    -   173: Electrically passivating and conductive film.    -   174: Electrically passivating interface layer.    -   175: n-type or p-type crystalline silicon wafer; thickness is        the range of w<300 um.    -   176: Electrically passivating interface layer.    -   177: Electrically passivating and conductive film.    -   178: Transparent and conductive film.    -   179: Back metal electrode.    -   179 a: Glass plate or transparent film carrying embedded metal        electrode.

The structures above are not mutually exclusive, and any feature of onestructure, can apply to any other structure herein, in accordance withthe present invention.

Fabrication Methods:

The following process flows are examples of methods to produce thestructures disclosed above; but other methods are possible withoutdeparting from the scope of the present invention. Initially, theincoming wafer is obtained free of surface damage, may be textured orotherwise modified in its geometrical shape, and has a clean surface. Asdiscussed above, and for the sake of simplicity, the geometrical surfaceshape of layer surfaces (e.g., surface texture such as pyramids, orother surface texture, can be formed on layer surfaces) is not portrayedin these drawings, however, it is understood that the geometrical shapeand/or surfaces may be textured in any shape beneficial for improvedsolar cell efficiency, and falls within the scope of the invention.

Subsequent processing steps can be as follows (the use of thedesignation such as “xx4” or any other similarly designated numberconnotes the analogous layers of any of the above structures of FIGS.1-18 ending in “4” or “4 a” such as 4, 14, 134, 4 a, 14 a, 134 a, etc):

-   -   Deposition or growth of the interface passivation layers xx4 and        xx6;    -   Deposition of layers xx3 and xx7;    -   Thermal treatment;    -   Optional deposition of layers xx2 and xx8 (including possibly a        low reflective index layer for a good internal mirror on the        back—refractive index basically smaller than 3.0, smaller than        2.6, smaller than 2.0, smaller than 1.5); and    -   Metallization.

In any of the structures above, the layers (e.g., xx2, xx3, xx4, xx6,xx7 and xx8) are electrically conductive are electrically conductive,i.e., the metallization can be placed directly on the outer layer. (Intypical high-efficiency solar cells this is not the case since surfacepassivation is usually done by materials that are also electricalinsulators.) This allows for innovative metallization schemes, forexample, the solar cells can be laminated into a module which has theelectrodes embedded in the glass or in the lamination sheets.Furthermore, conductive sheets can be applied to mechanically strengthenthe cells. Another way of metallization could involve the deposition ofthin lines of metals. Due to the conductivity of the surfaces, therequirements on the metal paste are reduced because they directlycontact the outer layers and do not need to etch through an insulatinglayer in order to contact the solar cell. Another example would be thedirect evaporation or sputtering of metal onto the conductive surfaces.

Most layers within the solar cell structures described above can bedeposited or grown with methods such as PECVD, APCVD, LPCVD, PVD,plating etc. For some layers and combinations of layers, innovativemethods of producing the layers and structures may be useful. Forexample, thermal oxidation or plasma deposition or plasma assistedoxidation can be used to form the interface passivating layer(s).

For example, in order to achieve a highly efficient solar cell with acost-effective production method, it is advantageous to deposit films ofdifferent characteristics only on one side. While this can be difficultto do, (e.g., for a standard tube furnace deposition of e.g. LPCVDdeposited polycrystalline silicon), a PECVD deposition can be done onone side of a wafer without deposition on the other side. PECVD toolsare available on an industrial scale but may only operate in temperatureregimes where amorphous or microcrystalline silicon layers can bedeposited. In the described cell structures, amorphous silicon layerscan be turned into polycrystalline silicon layers by thermal treatment.This also holds true for doped amorphous silicon layers or compounds ofamorphous silicon carbides, etc. This crystallization negatively affectsthe passivation quality of the silicon/amorphous silicon interface layer(if it exists in the cell structure). However, layers xx4 and xx6 bufferthe wafer surface from the crystallized polysilicon layer. Therefore,the interface is still passivated after the thermal treatment and thelayer systems are stable at the thermal-treatment temperature.

In accordance with the present invention, during the crystallizationprocess many properties of the layer change: Donors or acceptors getactivated, the optical transmission increases, hydrogen effuses from thelayer. The thermal treatment may activate doping atoms in the compoundand result in diffusion of dopant atoms into a substrate wafer toprovide a high-low junction or a p-n junction.

In accordance with the present invention, good passivation of layers xx4and xx6 persists and/or improves after high temperature thermaltreatment. Passivation may be adequate after deposition, but the hightemperature thermal treatment can improve its character. The passivationis temperature-stable (from 500° C., or 600° C., or 700° C., to 1100° C.or more) because of the composition of the layers. Thermal treatment at500° C. or more therefore comprises an aspect of the present invention.Other potential benefits of the structure may include: thermal treatmentmay not modify the crystallinity of the silicon substrate, at least atthe interface, because the first interface layer is amorphous SiO₂,and/or because the conductive layer is SiC. Therefore another aspect ofthe present invention contemplates providing a thermal treatment withoutmodifying the crystallinity of the silicon substrate, and/or that theinterface passivation layer acts as a buffer for re-crystallizationduring thermal treatment.

If the layer composition is chosen correctly, a layer deposited in asingle process can split into two (or more) layers. The incorporatedoxygen in the amorphous deposited layer migrates toward the siliconinterface and a thin oxide can be grown. If this mechanism is exploitedby the use of oxide containing films xx3 and xx7, the passivatinginterface layers xx4 and xx6 do not need to be produced prior to layersxx3 and xx7, therefore all described structures can also work withoutlayers xx4 and xx6. At the same time the film crystallizes and dopantsmay be activated. This effect can be employed to create structures suchas cells 90 and 140 disclosed above, in a very short process flow, butit is not restricted to this application. For that reason layers xx3 andxx7 in all structures can be used to employ this mechanism if theycontain a low amount of oxygen and the list of examples is expanded bythe same layers containing oxygen.

In case the passivating interface layers xx4 and xx6 and the highlydoped layers xx3 and xx7 were deposited or grown with built-in stress,or the thermal treatment for crystallization described above createsstress, this can negatively affect the passivation properties of thewafer surface xx5. In order to prevent this negative effect, and withreference to the partial cross sectional view of solar cell 180 of FIG.18 a thin silicon film 1831 and 1871 can be deposited on top of thepassivation films 184 and 186, to act as a buffer layer. FIG. 18illustrates this concept of a silicon buffer layers 1831 and 1871between the passivation layers 184 and 186 and the highly doped layers183 a, and 187 a, respectively. This concept is particularly beneficialfor cells 90 and 140 disclosed above, but its application is not limitedto these structures.

This silicon buffer layer can be, for example, undoped polysilicon. Inthis case, since the film can be deposited on both sides, a standardtube furnace can be used. In a process sequence where the passivationlayers 184 and 186 are a thin thermal oxide, the process of oxidationcan directly be followed by the deposition of polycrystalline silicon,in the same furnace but a different tube (saving handling of wafers) oreven in the same tube. The doping needed for passivation can be producedby driving dopants incorporated in films 183 a and 187 a, with thetemperature treatment used for crystallization at the same time drivesthe dopants from layers 183 a and 187 a into layers 1831 and 1871respectively, making them passivating and conductive. The allowedthickness of the buffer layer depends on the doping level of the dopedlayers which are deposited on top as well as on the time/temperatureprofile that is used for crystallization of this doped top layer. Theundoped layer is doped during this thermal treatment by the doped layers183 a and 187 a. The buffer layers 1831 and 1871 can also be composed ofmultiple silicon layers.

Another effect of the thermal treatment is the re-organization of thepassivating interface layers 184 and 186. Depending on their thickness,the thermal treatment and the layers over them, these layers shrink andvia-holes open (e.g., perforation occurs) such that the adjacent layers1831 and 1871 can make contact to the wafer 185 directly. A very smallfraction of the interface allows the carriers to bypass the layers 184and 186. If the thermal treatment is chosen in a way that no orinsufficient via-holes open up, the layers 184 and 186 need to be thinenough to allow for tunneling of the carriers.

Other aspects of the present invention include improved methods ofmetallization fabrication. In one example, metallization for any of theabove structures can be formed in accordance with previously filed U.S.Provisional Application entitled “Method for Forming Structures in aSolar Cell,” filed 21 Apr. 2009 and assigned application No. 61/171,187;and to commonly-assigned, co-filed International Patent Applicationentitled “Method for Forming Structures in a Solar Cell,” filed asAttorney Docket No. 3304.002AWO and assigned application No. ______.Each of these Applications is hereby incorporated by reference herein inits entirety. According to these Applications, metallization may beformed according to a method of forming a conductivecontact/heterocontact pattern on a surface of solar cell, includingforming a thin conductive layer over at least one lower layer of thesolar cell, and ablating a majority of the thin conductive layer using alaser beam, thereby leaving behind the conductive contact/heterocontactpattern. A self-aligned metallization may be formed on the conductivecontact pattern. The lower layer may include a passivation and/orantireflective layer beneath the thin conductive layer, wherein theconductive contact pattern forms an electrical contact through the atleast one lower layer to a semiconductor layer of the solar cell.

In another example, metallization for any of the above structures can beformed in accordance with previously filed U.S. Provisional Applicationentitled “Localized Metal Contacts By Localized Laser Assisted ReductionOf Metal-Ions In Functional Films, And Solar Cell Applications Thereof,”filed 22 Apr. 2009 and assigned application No. 61/171,491; and tocommonly-assigned, co-filed International Patent Application entitled“Localized Metal Contacts By Localized Laser Assisted Conversion OfFunctional Films In Solar Cells,” filed as Attorney Docket No.3304.003AWO and assigned application No. ______. Each of theseApplications is hereby incorporated by reference herein in its entirety.According to these Applications, metallization may be formed accordingto a method of forming at least one electrical contact in a layer of asolar cell, including forming a layer in the solar cell comprising amaterial which can be selectively modified to electrical contacts uponlaser irradiation; and applying selective laser irradiation to at leastone area of the layer to thereby form at least one electrical contact inthe area of the layer. A remaining region of the layer may comprise afunctional layer of the solar cell and need not be removed; e.g., atransparent, conductive film, and anti-reflective film, and/orpassivation as disclosed above.

The present invention extends to any one or a combination of the solarcell structures disclosed above, including generally a centralsubstrate, conductive layer(s), antireflection layers(s), passivationlayer(s) and/or electrode(s). The structures above are not mutuallyexclusive, and any feature of one structure, can apply to any otherstructure herein, in accordance with the present invention.

The present invention includes methods of fabricating any of thesestructures, including: providing a wafer as a central substrate;deposition or growth of interface passivation layers xx4 and xx6 overthe substrate; deposition of conductive layers xx3 and xx7 over thepassivation layers; providing thermal treatment; optional deposition ofantireflective layers xx2 and xx8 (including possibly a low reflectiveindex layer for a good internal mirror on the back); and providingmetallization as electrodes.

In one embodiment the present invention comprises applying a heattreatment to produce a multifunctional film which separates into asurface passivating interface layer and a highly doped polycrystallinepassivation layer with high transparency.

In one embodiment the present invention comprises depositing anamorphous, silicon containing compound and using a heat treatment inorder to initiate crystallization into a polycrystalline film.

In one embodiment the present invention comprises depositing anamorphous, silicon containing compound and using a heat treatment whichleads to a crystallization of the film and increases the opticaltransmissivity.

In one embodiment the present invention comprises depositing anamorphous, silicon containing compound and using a heat treatment inorder to activate doping atoms in the compound.

In one embodiment, the present invention comprises depositing anamorphous, silicon containing compound and using a thermal treatmentgreater than 500° C. in order to activate doping atoms in the compoundand result in diffusion of dopant atoms into a substrate wafer toprovide a high-low junction or a p-n junction.

One or more of the process control aspects of the present invention canbe included in an article of manufacture (e.g., one or more computerprogram products) having, for instance, computer usable media. The mediahas embodied therein, for instance, computer readable program code meansfor providing and facilitating the capabilities of the presentinvention. The article of manufacture can be included as a part of acomputer system or sold separately.

Additionally, at least one program storage device readable by a machineembodying at least one program of instructions executable by the machineto perform the capabilities of the present invention can be provided.

The flow diagrams and steps depicted herein are just examples. There maybe many variations to these diagrams or the steps (or operations)described therein without departing from the spirit of the invention.For instance, the steps may be performed in a differing order, or stepsmay be added, deleted or modified. All of these variations areconsidered a part of the claimed invention.

Although preferred embodiments have been depicted and described indetail herein, it will be apparent to those skilled in the relevant artthat various modifications, additions, substitutions and the like can bemade without departing from the spirit of the invention and these aretherefore considered to be within the scope of the invention as definedin the following claims.

1. A method of fabricating a solar cell, comprising: providing a waferas a central substrate; depositing or growing at least one interfacepassivation layer over the substrate; depositing at least one conductivelayer over the passivation layer(s); providing thermal treatment; andproviding metallization as electrodes.
 2. The method of claim 1, furthercomprising depositing at least one antireflective layer, and/or a lowreflective index layer forming a good internal mirror on a back of thesolar cell.
 3. A method of forming a solar cell, comprising applying aheat treatment to produce a multifunctional film which separates into asurface passivating interface layer and a highly doped polycrystallinepassivation layer with high transparency.
 4. The method of claim 3,wherein the heat treatment results in perforation of the passivationinterface layer to thereby allow carrier transport therethrough.
 5. Themethod of claim 1, wherein the passivation layer(s) comprises anamorphous, silicon containing compound, and wherein the thermaltreatment initiates crystallization into a polycrystalline film.
 6. Themethod of claim 1, wherein the passivation layer(s) comprises anamorphous, silicon containing compound and wherein the thermal treatmentleads to a crystallization of the film and increases the opticaltransmissivity.
 7. The method of claim 1, wherein the passivationlayer(s) comprises an amorphous, silicon containing compound and whereinthe thermal treatment activates doping atoms in the compound.
 8. Amethod of forming a solar cell, comprising depositing an amorphous,silicon containing compound and using a thermal treatment greater than500° C. to activate doping atoms in the compound and result in diffusionof dopant atoms into a substrate to provide a high-low junction or a p-njunction.
 9. A solar cell formed according to the method of claim
 1. 10.A solar cell, comprising: a wafer as a central substrate; at least oneinterface passivation layer over the substrate formed using thermaltreatment; at least one conductive layer over the passivation layer(s);and metallization as electrodes.
 11. The solar cell of claim 10, furthercomprising at least one antireflective layer, and/or a low reflectiveindex layer forming a good internal mirror on a back of the solar cell.12. The solar cell of claim 10, wherein the interface passivationlayer(s) comprises an amorphous, silicon containing compound, andwherein the thermal treatment initiates crystallization into apolycrystalline film.
 13. The solar cell of claim 10, wherein theinterface passivation layer(s) comprises an amorphous, siliconcontaining compound, and wherein the thermal treatment leads to acrystallization of the film and increases the optical transmissivity.14. The solar cell of claim 10, wherein the interface passivationlayer(s) comprises an amorphous, silicon containing compound, andwherein the thermal treatment activates doping atoms in the compound.15. A solar cell, comprising a multifunctional film thermally treated toseparate into a surface passivating interface layer and a highly dopedpolycrystalline passivation layer with high transparency.
 16. The solarcell of claim 15, wherein the passivating interface layer comprisesperforations to allow carrier transport therethrough, the perforationsformed by the thermal treatment of the multifunctional film.
 17. A solarcell, comprising an amorphous, silicon containing compound treated usinga thermal treatment greater than 500° C. to activate doping atoms in thecompound thereby resulting in diffusion of dopant atoms into a substrateto provide a high-low junction or a p-n junction.